Process for rigid polyurethane foams

ABSTRACT

Process for the preparation of a rigid polyurethane or urethane-modified polyisocyanurate foam in the presence of a blowing agent mixture comprising cyclopentane and a certain amounts of other organic compounds as co-blowing agenrts, the the saturated vapour pressure of said co-blowing agents in bar at T use  (v.p.) complying with the following equation (I) ##EQU1## wherein C is the mole % of said co-blowing agent in gaseous form on the total blowing agent mixture in the gaseous phase after foaming and T use  is the temperature in °K. at which the foam is used.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/113,146, filed Aug. 27, 1993, now abandoned.

DESCRIPTION

This invention relates to processes for the preparation of rigidpolyurethane or urethane-modified polyisocyanurate foams, to foamsprepared thereby, and to novel compositions useful in the process.

Rigid polyurethane and urethane-modified polyisocyanurate foams are ingeneral prepared by reacting the appropriate polyisocyanate andisocyanate-reactive compound (usually a polyol) in the presence of ablowing agent. One use of such foams is as a thermal insulation mediumas for example in the construction of refrigerated storage devices. Thethermal insulating properties of rigid foams are dependent upon a numberof factors including, for closed cell rigid foams, the cell size and thethermal conductivity of the contents of the cells.

A class of materials which has been widely used as blowing agent in theproduction of polyurethane and urethane-modified polyisocyanurate foamsare the fully halogenated chlorofluorocarbons, and in particulartrichlorofluoromethane (CFC-11). The exceptionally low thermalconductivity of these blowing agents, and in particular of CFC-11, hasenabled the preparation of rigid foams having very effective insulationproperties. Recent concern over the potential of chlorofluorocarbons tocause depletion of ozone in the atmosphere has led to an urgent need todevelop reaction systems in which chlorofluorocarbon blowing agents arereplaced by alternative materials which are environmentally acceptableand which also produce foams having the necessary properties for themany applications in which they are used.

A class of materials which have been used as alternative blowing agentswith zero ozone depletion potential are alkanes and cycloalkanes such asn-pentane, isopentane and cyclopentane. Especially cyclopentane ispreferred in view of its lower thermal conductivity. The use ofcyclopentane as blowing agent in rigid polyurethane or urethane-modifiedpolyisocyanurate foams is described in U.S. Pat. No. 5,096,933 andfurther in U.S. Pat. No. 5,114,986, U.S. Pat. No. 5,166,182, U.S. Pat.No. 4,795,763, U.S. Pat. No. 4,898,893, U.S. Pat. No. 5,026,502,European Patent Application No. 389011 and PCT patent application Ser.No. 92/16573.

A disadvantage of rigid polyurethane or urethane-modifiedpolyisocyanurate foams blown with cyclopentane is their poor dimensionalstability especially at temperatures below 20° C.: a freshly made foamshows substantial shrinking within the next 24 hours and also within thefollowing weeks. Further the aged thermal insulation properties (this isthe thermal insulation loss with time) of such foams is unsatisfactory,particularly at lower temperatures such as 10° C. which is the averagetemperature of a refrigerator and especially at -10° C. which is theaverage temperature of a freezer.

It is an object of the present invention to provide cyclopentane blownrigid polyurethane or urethane-modified polyisocyanurate foams havingimproved dimensional stabilities. It is a further object of the presentinvention to provide cyclopentane blown rigid polyurethane orurethane-modified polyisocyanurate foams having improved aged thermalinsulation properties. Another further object of the present inventionis to provide cyclopentane blown rigid polyurethane or urethane-modifiedpolyisocyanurate foams having improved dimensional stabilities andimproved aged thermal insulation properties without detrimentallyaffecting the initial thermal insulation of the foam.

These objects are met by using in the process of making rigidpolyurethane or urethane-modified polyisocyanurate foams frompolyisocyanates and isocyanate-reactive components besides cyclopentaneas blowing agent certain amounts of other organic compounds asco-blowing agents, the saturated vapour pressure of said co-blowingagents in bar at T_(use) (v.p.) complying with the following equation(I) ##EQU2## wherein C is the mole % of said compound in gaseous form onthe total blowing agent mixture in the gaseous phase after foaming andT_(use) is the temperature in °K. at which the foam is used.

The saturated vapour pressure at T_(use) can be calculated from theboiling point of the co-blowing agent, its molecular heat of evaporationand T_(use) by using the Clausius-Clapeyron equation.

C, the mole % of the co-blowing agent on the total gaseous blowing agentmixture after foaming is determined assuming that no condensation of theblowing agent mixture in the foam cells takes place.

According to the present invention cyclopentane is used in combinationwith one or more other organic co-blowing agents the saturated vapourpressure of each of said co-blowing agents complying with the aboveequation (I).

Preferred classes of compounds for use as co-blowing agent according tothe present invention are (cyclo)alkanes, hydrofluorocarbons,hydrochlorofluorocarbons, fluorocarbons, fluorinated ethers, alkenes,alkynes and noble gases. Especially preferred classes are alkanes andhydrofluorocarbons.

Examples of suitable (cyclo)alkanes for use according to the presentinvention include isopentane, n-pentane, neopentane, n-butane,cyclobutane, methylcyclobutane, isobutane, propane, cyclopropane,methylcyclopropane, n-hexane, 3-methylpentane, 2-methylpentane,cyclohexane, methylcyclopentane, n-heptane, 2-methylheptane,3-ethylpentane, 2,2,3-trimethylbutane, 2,2-dimethylpentane,cycloheptane, methylcyclohexane and 2,3-dimethylbutane.

Examples of suitable hydrofluorocarbons for use according to the presentinvention include 1,1,1,2-tetrafluoroethane (HFC 134a),1,1,1,4,4,4hexafluorobutane (HFC 356), pentafluoroethane (HFC 125),1,1-difluoroethane (HFC 152a), trifluoromethane (HFC 23),difluoromethane (HFC 32), trifluoroethane (HFC 143) and fluoromethane.

Examples of suitable hydrochlorofluorocarbons for use according to thepresent invention include chlorodifluoromethane (HCFC 22),1-chloro-1,1-difluoroethane (HCFC 142b),1,1,1-trifluoro-2,2-dichloroethane (HCFC 123) and dichlorofluoromethane(HCFC 21).

Examples of suitable fluorocarbons for use according to the presentinvention include perfluoromethane (R 14), perfluorocyclobutane,perfluorobutane, perfluoroethane and perfluoropropane.

Examples of suitable fluorinated ethers for use according to the presentinvention include bis-(trifluoromethyl) ether, trifluoromethyldifluoromethyl ether, methyl fluoromethyl ether, methyl trifluoromethylether, bis(difluoromethyl) ether, fluoromethyl difluoromethyl ether,methyl difluoromethyl ether, bis-(fluoromethyl) ether,2,2,2-trifluoroethyl difluoromethyl ether, pentafluoroethyltrifluoromethyl ether, pentafluoroethyl difluoromethyl ether,1,1,2,2-tetrafluoroethyl difluoromethyl ether, 1,2,2,2-tetrafluoroethylfluoromethyl ether, 1,2,2-trifluoroethyl difluoromethyl ether,1,1-difluoroethyl methyl ether and 1,1,1,3,3,3-hexafluoroprop-2-ylfluoromethyl ether.

Examples of suitable alkenes for use according to the present inventioninclude ethylene, propylene, 1-butene, 2-butene, propadiene, butadieneand methylbutene.

Examples of suitable alkynes for use according to the present inventioninclude acetylene and methylacetylene.

Examples of suitable noble gases for use according to the presentinvention include krypton, argon and xenon.

Preferred compounds for use as co-blowing agents according to thepresent invention are isopentane, n-pentane and HFC 134a and aparticularly preferred compound is isopentane.

By using compounds of which the saturated vapour pressure complies withthe above equation (I) as blowing agents in addition to cyclopentane,rigid polyurethane foams are obtained that show improved dimensionalstability at T_(use) compared to foams blown with cyclopentane alone.

Thus for example by using isopentane in an amount up to 75 mole % of thegaseous blowing agent mixture in addition to cyclopentane, foams withimproved dimensional stability at 10° C. (average temperature at which arigid polyurethane foam is used in refrigeration) are obtained.Similarly by using n-pentane in an amount up to 55 mole % of the gaseousblowing agent mixture in addition to cyclopentane, foams with improveddimensional stability at 10° C. are obtained.

According to a preferred embodiment of the present invention water orother carbon dioxide-evolving compounds are used together with theblowing agent mixture according to the present invention. Water isusually added to improve the flow of the reaction mixture. Where wateris used as chemical co-blowing agent typical amounts are in the rangefrom 0.5 to 3% by weight based on the isocyanate-reactive compound.

The optimum molar ratio cyclopentane/co-blowing agent according to thepresent invention will therefore depend not only on T_(use) and thevapour pressure of the co-blowing agent but also on the free risedensity and on the amount of water that is used.

The optimum molar ratio cyclopentane/isopentane for a polyurethane foamwith a free rise density of 22 kg/m³ and a water amount of 0.5 pbw onthe isocyanate-reactive component is between 15/85 and 40/60 for T_(use)being 10° C. and between 15/85 and 65/35 for T_(use) being -10° C. Theoptimum molar ratio cyclopentane/isopentane for a polyurethane foam witha free rise density of 22 kg/m³ and a water amount of 2 pbw on theisocyanate-reactive component is between 5/95 and 45/55 for T_(use)being 10° C. and between 20/80 and 50/50 for T_(use) being -10° C. Theoptimum molar ratio cyclopentane/isopentane for a polyurethane foam witha free rise density of 22 kg/m³ and a water amount of 3 pbw on theisocyanate-reactive component is between 5/95 and 75/25 for T_(use)being 10° C. and between 25/75 and 35/65 for T_(use) being -10° C.

In addition to the dimensional stability of the foams obtained by usingthe blowing agent mixture according to the present invention the agedthermal insulation properties of such foams can be improved. This willdepend on the diffusion characteristics and thermal insulationproperties of the co-blowing agent. The optimal amount of the co-blowingagent will therefore also depend on these characteristics of theco-blowing agent.

In particular when certain amounts of isopentane or n-pentane are usedin combination with cyclopentane as blowing agent, rigid polyurethane orurethane-modified polyisocyanurate foams are obtained with improveddimensional stability and improved aged thermal insulation propertiescompared to foams blown with cyclopentane alone. And surprisingly theinitial thermal conductivity of said cyclopentane/isopentane orn-pentane blown foams is not significantly higher than the initialthermal conductivity of cyclopentane blown foams although the thermalinsulation properties of isopentane and n-pentane are worse than thethermal insulation properties of cyclopentane. Further isopentane andn-pentane are considerably cheaper than cyclopentane. Thus bysubstituting part of the cyclopentane by the worse insulating materialsisopentane or n-pentane foams are obtained with comparable initialthermal conductivity at lower cost and moreover the dimensionalstability and the aged thermal insulation properties of such foams areimproved compared to cyclopentane blown foams.

When isopentane or n-pentane is used in combination with cyclopentanethe preferred molar ratio cyclopentane/iso- or n-pentane for T_(use)being 10° C. is between 90/10 and 30/70, preferably between 80/20 and30/70 and more preferably between 70/30 and 40/60 and most preferablybetween 65/35 and 45/55 in order to obtain rigid polyurethane foams withimproved dimensional stability and improved aged thermal insulationproperties without significantly affecting the initial thermalconductivity.

In addition to the cyclopentane and the co-blowing agent(s) complyingwith the above equation (I) the blowing agent mixture according to thepresent invention may also contain up to 20 mole % of the total blowingagent mixture of other physical blowing agents not complying withequation (I). This is especially true since the commercially availablecyclopentane is usually a technical (70 to 80%) grade comprising smallamounts of other hydrocarbons rather than a pure grade. Suitablephysical blowing agents include those well known and described in theart, for example hydrocarbons, dialkyl ethers, alkyl alkanoates,aliphatic and cycloaliphatic hydrofluorocarbons,hydrochlorofluorocarbons, chlorofluorocarbons, hydrochlorocarbons andfluorine-containing ethers.

The total quantity of blowing agent to be used in a reaction system forproducing cellular polymeric materials will be readily determined bythose skilled in the art, but will typically be from 2 to 25% by weightbased on the total reaction system.

In order to reduce the cell sizes of the foam and accordingly to improvethe thermal insulation properties an inert, insoluble fluorinatedcompound may be used in the foam-forming process according to thepresent invention. Such inert, insoluble fluorinated compounds includeany of those disclosed in U.S. Pat. No. 4,981,879, U.S. Pat. No.5,034,424, U.S. Pat. No. 4,972,002 and European Patent Application No.0508649. Certain of said inert, insoluble fluorinated compounds suitablefor use in the process of the invention may themselves act as blowingagents under the conditions pertaining to the foam-forming reaction,particularly where their boiling point is lower than the exothermtemperature achieved by the reaction mixture.

Suitable organic polyisocyanates for use in the process of the presentinvention include any of those known in the art for the preparation ofrigid polyurethane or urethane-modified polyisocyanurate foams, and inparticular the aromatic polyisocyanates such as diphenylmethanediisocyanate in the form of its 2,4'-, 2,2'- and 4,4'-isomers andmixtures thereof, the mixtures of diphenylmethane diisocyanates (MDI)and oligomers thereof known in the art as "crude" or polymeric MDI(polymethylene polyphenylene polyisocyanates) having an isocyanatefunctionality of greater than 2, toluene diisocyanate in the form of its2,4- and 2,6-isomers and mixtures thereof, 1,5-naphthalene diisocyanateand 1,4-diisocyanatobenzene. Other organic polyisocyanates which may bementioned include the aliphatic diisocyanates such as isophoronediisocyanate, 1,6-diisocyanatohexane and4,4'-diisocyanatodicyclohexylmethane.

Polyfunctional isocyanate-reactive compositions with which thepolyisocyanate composition can be reacted to form rigid polyurethane orurethane-modified polyisocyanurate foams include any of those known inthe art for that purpose. Of particular importance for the preparationof rigid foams are polyols and polyol mixtures having average hydroxylnumbers of from 300 to 1000, especially from 300 to 700 mg KOH/g, andhydroxyl functionalities of from 2 to 8, especially from 3 to 8.Suitable polyols have been fully described in the prior art and includereaction products of alkylene oxides, for example ethylene oxide and/orpropylene oxide, with initiators containing from 2 to 8 active hydrogenatoms per molecule. Suitable initiators include: polyols, for exampleglycerol, trimethylolpropane, triethanolamine, pentaerythritol, sorbitoland sucrose; polyamines, for example ethylene diamine, tolylene diamine,diaminodiphenylmethane and polymethylene polyphenylene polyamines; andaminoalcohols, for example ethanolamine and diethanolamine; and mixturesof such initiators. Preferred initiators for use in the presentinvention are diaminodiphenyimethane and polymethylene polyphenylenepolyamines. Foams made from polyols initiated by these compounds showimproved dimensional stability compared to foams made from polyolsinitiated by other conventional compounds such as sucrose. Othersuitable polymeric polyols include polyesters obtained by thecondensation of appropriate proportions of glycols and higherfunctionality polyols with dicarboxylic or polycarboxylic acids. Stillfurther suitable polymeric polyols include hydroxyl terminatedpolythioethers, polyamides, polyesteramides, polycarbonates,polyacetals, polyolefins and polysiloxanes.

The quantities of the polyisocyanate compositions and the polyfunctionalisocyanate-reactive compositions to be reacted will depend upon thenature of the rigid polyurethane or urethane-modified polyisocyanuratefoam to be produced and will be readily determined by those skilled inthe art.

In addition to the polyisocyanate and polyfunctional isocyanate-reactivecompositions and the blowing agent mixture of the present invention, thefoam-forming reaction mixture will commonly contain one or more otherauxiliaries or additives conventional to formulations for the productionof rigid polyurethane and urethane-modified polyisocyanurate foams. Suchoptional additives include crosslinking agents, for examples lowmolecular weight polyols such as triethanolamine, foam-stabilisingagents or surfactants, for example siloxane-oxyalkylene copolymers,urethane catalysts, for example tin compounds such as stannous octoateor dibutyltin dilaurate or tertiary amines such asdimethylcyclohexylamine or triethylene diamine, and fire retardants, forexample halogenated alkyl phosphates such as tris chloropropylphosphate.

In operating the process for making rigid foams according to theinvention, the known one-shot, prepolymer or semi-prepolymer techniquesmay be used together with conventional mixing methods and the rigid foammay be produced in the form of slabstock, mouldings, cavity fillings,sprayed foam, frothed foam or laminates with other materials such ashardboard, plasterboard, plastics, paper or metal.

It is convenient in many applications to provide the components forpolyurethane production in pre-blended formulations based on each of theprimary polyisocyanate and isocyanate-reactive components. Inparticular, many reaction systems employ a polyisocyanate-reactivecomposition which contains the major additives such as the blowing agentand the catalyst in addition to the polyisocyanate-reactive component orcomponents.

Therefore the present invention also provides a polyisocyanatecomposition or a polyisocyanate-reactive composition comprising asblowing agents cyclopentane and certain amounts of other organiccompounds of which the saturated vapour pressure at T_(use) complieswith the following equation (I) ##EQU3## wherein v.p. is the saturatedvapour pressure of said compound in bar at T_(use), C is the mole % ofsaid compound in gaseous form on the total blowing agent mixture in thegaseous phase after foaming and T_(use) is the temperature in °K. atwhich the foam is used.

The various aspects of this invention are illustrated, but not limitedby the following examples.

The following formulation and reaction components are referred to in theexamples:

Daltolac XR159:A polyether polyol from Imperial Chemical Industries

Daltolac XR144:A polyether polyol from Imperial Chemical Industries

Daltolac R 180:A polyether polyol from Imperial Chemical Industries

Daltolac R 260:A polyether polyol from Imperial Chemical Industries

Catalyst SFC: A tertiary amine catalyst from Imperial ChemicalIndustries

Niax A1: A tertiary amine catalyst from Union Carbide

DC 193: A silicone surfactant from Air Products

RS 201: A surfactant from Union Carbide

Cyclopentane A:A 78% grade cyclopentane from Exxon

Cyclopentane B:A 98% grade cyclopentane from Shell

Isopentane: A 98% grade isopentane from Janssen

Suprasec DNR: A polymeric MDI from imperial Chemical Industries

Daitolac and Suprasec are trademarks from Imperial Chemical IndustriesPLC.

EXAMPLE 1

Rigid polyurethane foams were prepared from the starting componentsgiven in table 1 (amounts in pbw). Characteristics of the foam-formingreaction profiles, density and thermal conductivity characteristics weredetermined. Table 1 records the cream time (time taken from the mixingof the two reactive compositions till the onset of the blowingreaction), the string time (time taken for the reaction mixture to reachthe transition point from fluid to cross-linked mass) and the end ofrise time, all assessed visually. Further the free rise density (densityof the foam after free expansion) and the core density (measuredaccording to DIN 53420 standard) of the foam is recorded. Thermalconductivity properties (lambda values) at a mean temperature of 10° C.were determined initially, after 6 days at 70° C., after 3 weeks at 70°C. and after 5 weeks at 70° C.

                                      TABLE 1                                     __________________________________________________________________________    FOAM              Comp 1                                                                             2   3   4   5   6                                      __________________________________________________________________________    XR 159       pbw  58.0 58.0                                                                              58.0                                                                              58.0                                                                              58.0                                                                              58.0                                   XR 144       pbw  42.0 42.0                                                                              42.0                                                                              42.0                                                                              42.0                                                                              42.0                                   SFC          pbw  1.4  1.4 1.4 1.4 1.4 1.4                                    Niax A1      pbw  0.1  0.1 0.1 0.1 0.1 0.1                                    DC 193       pbw  3.0  3.0 3.0 3.0 3.0 3.0                                    Water        pbw  2.0  2.0 2.0 2.0 2.0 2.0                                    Cyclopentane A                                                                             pbw  15.0 13.5                                                                              12.0                                                                              10.5                                                                              9.0 7.5                                    Isopentane   pbw  --   1.5 3.0 4.5 6.0 7.5                                    DNR          pbw  140.0                                                                              140.0                                                                             140.0                                                                             140.0                                                                             140.0                                                                             140.0                                  Reaction Profile                                                              Cream time   sec  10   10  10  10  10  10                                     String time  sec  54   54  54  55  56  56                                     End of rise time                                                                           sec  135  135 135 135 135 135                                    Free rise density                                                                          kg/m.sup.3                                                                         24.1 24.4                                                                              24.9                                                                              24.5                                                                              24.4                                                                              24.0                                   Thermal Conductivity                                                          Core Density kg/m.sup.3                                                                         28.25                                                                              27.75                                                                             28.45                                                                             28.75                                                                             29.0                                                                              29.3                                   Initial Lambda                                                                             mW/mK                                                                              21.2 21.6                                                                              21.9                                                                              21.9                                                                              21.85                                                                             21.8                                   Lambda 6d/70° C.                                                                    mW/mK                                                                              24.9 25.3                                                                              25.25                                                                             24.35                                                                             24.6                                                                              24.45                                  Lambda Increase after 6d                                                                   mW/mK                                                                              +3.7 +3.7                                                                              +3.35                                                                             +2.45                                                                             +2.75                                                                             +2.65                                  Lambda 3w/70° C.                                                                    mW/mK                                                                              26.85                                                                              26.95                                                                             26.8                                                                              26.3                                                                              26.15                                                                             25.85                                  Lambda Increase after 3w                                                                   mW/mK                                                                              +5.65                                                                              +5.35                                                                             +4.9                                                                              +4.4                                                                              +4.3                                                                              +4.05                                  Lambda 5w/70° C.                                                                    mW/mK                                                                              27.45                                                                              27.25                                                                             27.25                                                                             26.6                                                                              26.6                                                                              26.45                                  Lambda Increase after 5w                                                                   mW/mK                                                                              +6.25                                                                              +5.65                                                                             +5.35                                                                             +4.7                                                                              +4.75                                                                             +4.65                                  __________________________________________________________________________

These results show that by using mixtures of cyclopentane and isopentanein molar ratios of cyclopentane/isopentane ranging from about 90/10 to40/60 (foams 2 to 6) foams are obtained with improved aged thermalinsulation properties without substantially affecting the initialthermal insulation compared to foams blown with cyclopentane alone(comparative foam 1).

EXAMPLE 2

Rigid polyurethane foams were prepared from the starting componentsgiven in table 2 (amounts in pbw). The dimensional stability of theresulting foams at -20° C. after 1 day and after 14 days was checked bymeasuring the variation in length, in width and in thickness (accordingto ISO 2796 standard). The results are presented in table 2.

                  TABLE 2                                                         ______________________________________                                        FOAM              Comp 7    8       9                                         ______________________________________                                        R 180         pbw     85        85    85                                      R 260         pbw     15        15    15                                      SFC           pbw     3.5       3.5   3.5                                     Niax A1       pbw     0.2       0.2   0.2                                     Water         pbw     2.0       2.0   2.0                                     RS 201        pbw     3.0       3.0   3.0                                     Cyclopentane A                                                                              pbw     15.0      10.5  7.5                                     Isopentane    pbw     --        4.5   7.5                                     DNR           pbw     140       140   140                                     Core Density  kg/m.sup.3                                                                            27.5      28.1  27.4                                    Dimensional stability                                                         after 1 day                                                                   length        %       -1.06     -3.54 -0.09                                   width         %       -1.07     3.42  0.18                                    thickness     %       0.76      0.53  0.63                                    Dimensional stability                                                         after 14 days                                                                 length        %       -4.60     -0.96 -0.44                                   width         %       -2.16     -1.19 0.06                                    thickness     %       0.43      0.62  -0.18                                   ______________________________________                                    

These results show that by using mixtures of cyclopentane and isopentanefoams are obtained with improved dimensional stabilities compared tofoams blown with cyclopentane alone (comparative foam 7).

EXAMPLE 3

Rigid polyurethane foams were prepared from the starting componentsgiven in table 3 (amounts in pbw). Density and thermal conductivitycharacteristics were determined. Thermal conductivity properties (lambdavalues) were determined initially, after 1 week at 70° C., after 3 weeksat 70° C. and after 5 weeks at 70° C.

                                      TABLE 3                                     __________________________________________________________________________    FOAM               Comp 10                                                                            11  Comp 12                                                                            13                                           __________________________________________________________________________    XR 144        pbw  42.0 42.0                                                                              --   --                                           XR 159        pbw  58.0 58.0                                                                              --   --                                           R 180         pbw  --   --  85   85                                           R 260         pbw  --   --  15   15                                           SFC           pbw  1.4  1.4 3.5  3.5                                          Niax A1       pbw  0.1  0.1 0.2  0.2                                          DC 193        pbw  3.0  3.0 --   --                                           RS 201        pbw  --   --  3.0  3.0                                          Water         pbw  2.0  2.0 2.0  2.0                                          Cyclopentane B                                                                              pbw  15.0 7.5 15.0 7.5                                          Isopentane    pbw  --   7.5 --   7.5                                          DNR           pbw  140.0                                                                              140.0                                                                             140.0                                                                              140.0                                        Core Density  kg/m.sup.3                                                                         34   29  35   31                                           Initial Lambda                                                                              mW/mK                                                                              21.1 21.5                                                                              21.5 22.1                                         Lambda 1w/70° C.                                                                     mW/mK                                                                              23.9 24.5                                                                              25.9 26.6                                         Lambda increase after 1w                                                                    mW/mK                                                                              2.8  3.0 4.4  4.5                                          Lambda 3w/70° C.                                                                     mW/mK                                                                              26.0 26.1                                                                              27.4 27.4                                         Lambda increase after 3w                                                                    mW/mK                                                                              4.9  4.6 5.9  5.3                                          Lambda 5w/70° C.                                                                     mW/mK                                                                              27.5 26.4                                                                              27.5 27.1                                         Lambda increase after 5w                                                                    mW/mK                                                                              6.4  4.9 6.0  5.0                                          __________________________________________________________________________

I claim:
 1. Process for the preparation of a rigid polyurethane orurethane-modified polyisocyanurate foam by reaction of a polyisocyanatecomposition with a polyfunctional isocyanate-reactive composition underfoam-forming conditions in the presence of a blowing agent mixturecomprising cyclopentane, characterised in that said blowing agentmixture further comprises certain amounts of other organic compounds asco-blowing agents, wherein said co-blowing agent is selected from thegroup consisting of an alkane, a cycloalkane and a hydrofluorocarbon,the saturated vapour pressure of said co-blowing agents in bar atT_(use) (v.p.) complying with the following Equation (I): ##EQU4##wherein C is the mole % of said co-blowing agent in gaseous form on thetotal blowing agent mixture in the gaseous phase after foaming andT_(use) is the temperature in °K. at which the foam is used.
 2. Processaccording to claim 1, wherein the co-blowing agent is isopentane orn-pentane or 1,1,1,2-tetrafluoroethane.
 3. Process according to claim 1,wherein water is also present in amounts ranging from 0.5 to 3% byweight based on the isocyanate-reactive compound.
 4. Process accordingto claim 2, wherein the molar ratio cyclopentane/isopentane is between90/10 and 30/70.
 5. Process according to claim 4, wherein the molarratio cyclopentane/isopentane is between 65/35 and 45/55.